The present invention relates to a gas concentration measuring method and a related composite gas sensor preferably used in an exhaust gas purification system employed in an internal combustion engine of an automotive vehicle.
Exhaust gases emitted from the internal combustion engines are main causes of the present-days serious air pollution. To suppress any harmful substances involved in the exhaust gas, the related laws and regulations have been becoming severe year by year.
In view of the foregoing, the air-fuel ratio of the gas mixture supplied to a combustion chamber of the internal combustion engine must be precisely controlled. Similarly, the ignition timing of the internal combustion engine must be carefully controlled. Furthermore, many of the internal combustion engines are equipped with catalytic converters to purify the exhaust gas.
According to the OBD-II regulation in the United States, each purification system is required to have the capability of judging whether or not the catalytic converter operates properly. To this end, an advanced exhaust gas purification system proposes to directly measure the harmful substances, i.e., NOx concentration, as well as precisely controlling the engine combustion.
In general, the catalytic converter is disposed in an exhaust gas passage of an internal combustion engine. When the catalytic converter deteriorates, an increased amount of NOx gas flows across the catalytic converter. In other words, the deterioration of the catalytic converter can be known by the increase amount of the NOx gas not trapped by the catalytic converter. Accordingly, an appropriate NOx sensor is provided downstream of the catalytic converter to detect the change of NOx concentration.